Insect olfaction refers to the function of chemical receptors that enable

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...

to detect and identify volatile compounds for

foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology that studies the foraging behavi ...

, predator avoidance, finding

mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...

partners (via

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

s) and locating

oviposition The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...

habitats. Thus, it is the most important sensation for insects. Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. For example, olfaction is essential for locating host plants and hunting prey in many species of insects, such as the

moth Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...

'' Deilephila elpenor'' and the

wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder ...

Polybia sericea ''Polybia sericea'' is a social, tropical wasp of the family Vespidae that can be found in South America. It founds its colonies by swarming migrations, and feeds on nectar and arthropods. ''P. sericea'' is medium in size, and has a dark-color ...

'', respectively. The two organs insects primarily use for detecting odors are the antennae and specialized mouth parts called the maxillary palps. However, a recent study has demonstrated the olfactory role of ovipositor in fig wasps. Inside of these olfactory organs there are neurons called olfactory receptor neurons which, as the name implies, house receptors for scent molecules in their cell membrane. The majority of

olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are ...

s typically reside in the

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

. These neurons can be very abundant; for example, ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...

'' flies have 2,600 olfactory sensory neurons. Insects are capable of smelling and differentiating between thousands of volatile compounds both sensitively and selectively. Sensitivity is how attuned the insect is to very small amounts of an odorant or small changes in the concentration of an odorant. Selectivity refers to the insects ability to tell one odorant apart from another. Among blood-feeding arthropods, these compounds are commonly broken into three classes: short chain

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...

s and low molecular weight nitrogenous compounds. Insects have been used as a model system to study mammal and especially human

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

. Yet, unlike vertebrates who use G protein coupled receptors ( GPCRs), insects express proteins including ORs ( olfactory receptors), GRs (gustatory receptors) and IRs ( ionotropic receptors) which are all

heteromeric A heteromer is something that consists of different parts; the antonym of homomeric. Examples are: Biology * Spinal neurons that pass over to the opposite side of the spinal cord. * A protein complex that contains two or more different polypeptides ...

ligand-gated ion channels Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in res ...

. A moth species in the order of

Lepidoptera Lepidoptera ( ) is an order of insects that includes butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 families and 46 superfamilies, 10 percent of the total described speci ...

known as the black cutworm moth ('' Agrotis ipsilon'') produces even more proteins including OBPs ( odorant-binding proteins), CSPs (chemosensory binding proteins), and SNMPs (sensory neuron membrane proteins) that help the moth recognize sex pheromones and odorants such as those released from host plants. Much like in vertebrates, axons from the sensory neurons converge into glomeruli, but differ in where the glomeruli are housed. In mammals they are located in the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex ...

s, but in insects they are in the

antennal lobe The antennal lobe is the primary (first order) olfactory brain area in insects. The antennal lobe is a sphere-shaped deutocerebral neuropil in the brain that receives input from the olfactory sensory neurons in the antennae and mouthparts. Functi ...

Evolutionary history

Olfaction is metabolically costly. Even more costly are more complex traits associated with it. Niven & Laughlin 2008 find high olfactory ''sensitivity'' to be energetically costly, and other research has found that the ability to ''change the final behavior'' that results from the stimulus is as well. The evolutionary

trade-off A trade-off (or tradeoff) is a situational decision that involves diminishing or losing one quality, quantity, or property of a set or design in return for gains in other aspects. In simple terms, a tradeoff is where one thing increases, and anot ...

s involved require even further study because most such research has been done under laboratory conditions with unrealistically reliable food.

Olfactory cascade

Sensory neurons in the antenna, maxillary palp, and labella generate odor-specific electrical signals called spikes (

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

s) in response to binding of odors to cell surface proteins like the olfactory receptors. The sensory neurons in the antenna and maxillary send this information via their

axons An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

to the

, while sensory neuron in the labella send this information via axons to the

subesophageal ganglion The suboesophageal ganglion (acronym: SOG; synonym: ''subesophageal ganglion'') of arthropods and in particular insects is part of the arthropod central nervous system (CNS). As indicated by its name, it is located ''below the'' ''oesophagus'', ins ...

.{{Cite journal, last1=Riabinina, first1=Olena, last2=Task, first2=Darya, last3=Marr, first3=Elizabeth, last4=Lin, first4=Chun-Chieh, last5=Alford, first5=Robert, last6=O'Brochta, first6=David A., last7=Potter, first7=Christopher J., date=2016-10-03, title=Organization of olfactory centres in the malaria mosquito Anopheles gambiae, journal=Nature Communications, volume=7, pages=13010, doi=10.1038/ncomms13010, pmc=5063964, pmid=27694947, bibcode=2016NatCo...713010R Inside the antennal lobe they synapse with other neurons in semidelineated (with membrane boundaries) structures called glomeruli. Specifically the process is as follows: first the odorant wafts towards an insect's antenna or maxillary palp which is covered with hair-like projections called

sensilla A sensillum (plural ''sensilla'') is an arthropod sensory organ protruding from the cuticle of exoskeleton, or sometimes lying within or beneath it. Sensilla appear as small hairs or pegs over an individual's body. Inside each sensillum there are ...

. The odorant then enters through tiny pores in the exoskeleton (or cuticle) of that sensillum and diffuses into the fluid between the cells called

extracellular fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a low ...

s. There the odorant molecule binds to an odorant binding protein which transports it to a receptor and co-receptor (Orco) team on the surface of the olfactory receptor neuron (ORN). This leads to the neuron firing an

down the axon. This signal is sent to the antennal lobe or subesophogeal ganglion of the insects brain where it can then integrate the information with other signals from other

. These ORNs are bipolar, on one end are the olfactory

dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the ...

with the receptors for the odors and on the other end are the axons that carry the

to the antennal lobe of the brain. The antennal lobes have two kinds of neurons,

projection neuron The projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord. In human neuroanatomy, bundles of axons (nerve fibers) called tracts, within the brain, can be cate ...

s (mostly excitatory) and local neurons (inhibitory, with some excitatory). The projection neurons send their axon terminals to a part of the insect brain called the mushroom bodies (important in regulating learned odor responses) and another part of the brain called the lateral horn (important in regulating innate odor responses). Both of these regions are part of the

protocerebrum The protocerebrum is the first segment of the panarthropod brain. Recent studies suggest that it comprises two regions. Region associated with the expression of ''six3'' ''six3'' is a transcription factor that marks the anteriormost part of ...

of the insect brain.

Research methods

Action potential recordings are conducted in three different ways electroantenograms, electropalpograms, and single sensillum recordings (SSR). In electroantenograms (EAG) and electropalpograms (EPG) the action potentials from the entire antenna or maxillary palp, respectively, is recorded. EAGs and EPGs provide an overall view of olfaction in the respective organ. During an SSR an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

is inserted into just one sensillum and the recording is made from only the ORNs which are contained within that sensillum, providing more detailed information^. Any of these methods can be combined with a high resolution

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substanc ...

to isolate volatile compounds from important animals or habitats. For example, this method could be used to determine which compound from a particular flower is the most attractive to a bee. Recordings from projection neurons show that in some insects there is strong specialization and discrimination for the odors presented by the ORNs. This is especially true for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection.

Repellents and attractants

Humans exploit the insect olfactory system to control agricultural and disease carrying pests. For some agricultural pests manufactured sex pheromones are placed in traps to capture adults before they can

oviposit The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...

(lay their eggs) leading to the hatching of their destructive larvae. While there are thousands of chemicals insects can detect there is a limited range that insects use as cues to move towards or away from the source of the odorant. The art of finding an attractant or repellent for a particular insect of interest is complicated and a long, intensive process. For example, using

s only attracts insects in their reproductive stage, a short period in their lives. While scents of food may be attractive to hungry insects they would not be effective in a field full of a crop that is palatable to that insect.

Situationally-dependent attractants / repellents

Insects use the same signal for many different uses depending on the situation this is called chemical parsimony. Situations that may change how an insect behaves in reaction to a scent are things like the concentration of the compound, the life stage of the insect, its mating status, other olfactory cues, the insects feeding state (hungry or full), the time of day, or even the insects body position. For example, Drosophila are very attracted to apple cider vinegar but in very high concentrations an additional olfactory receptor (that has low affinity for the vinegar, Or85a) is activated which changes the fly’s behavior from attraction to aversion. These different behaviors to the same cue is called behavioral plasticity.

Carbon dioxide

Many insects are capable of detecting very minute changes in the concentration of CO₂. While CO₂ has been found to be an attractant in every arthropod studied and it is very important in mosquito monitoring and control, even this stereotyped reaction can be plastic. Drosophila avoid CO₂ when walking but move towards it when in flight.

DEET

Many insects (and other arthropods) have been shown to avoid areas containing N,N-diethyl-3-methylbenzamide or

DEET ''N'',''N''-Diethyl-''meta''-toluamide, also called DEET () or diethyltoluamide, is the most common active ingredient in insect repellents. It is a slightly yellow oil intended to be applied to the skin or to clothing and provides protection a ...

. They innately avoid DEET, likely because it is a “confusant” that stimulates gustatory, ionotropic, and olfactory receptors and “distorts” other odorants interaction with those receptors.

References

Olfaction Insect physiology